,

. i !

- 219 -

SOME PROBLEMS I N

ANALYSIS OF SOILS IN ARID AREAS

J . O . J o b

Ana lys i s of s o i l s of a r i d zones p r e s e n t s , i n a d d i t i o n t o t h e d i f f i -

c u l t i e s u s u a l l y encountered, some p e c u l i a r s o u r c e s of inaccuracy. Various

chemical and p h y s i c a l f a c t o r s , a lmost a l l of them due t o more o r less

s o l u b l e s a l t s , i n t e r a c t t o a f f e c t t h e f i n a l value. I n a d d i t i o n t o inac-

curacy, i n t e r f e r e n c e s may t a k e two forms: chemical i n t e r f e r e n c e due t o

s o l u b i l i t y of s a l t s o r phys ica l i n t e r f e r e n c e due t o more o r less c r y s t a l -

l i z e d forms of s p a r i n g l y s o l u b l e s a l t s .

s o i l i s brought i n t o c o n t a c t w i th an aqueous s o l u t i o n , i.e. duking a l l

chemical a n a l y s i s of s o i l s .

calcium ca rbona te and gypsum are major components of t h e s o i l .

o f t h e i r p h y s i c a l parameters g i v e d i f f e r e n t r e s u l t s whether t h e s e s a l t s

r e a c t as s o l u b l e o r i n s o l u b l e p a r t i c l e s .

The f i r s t t y p e i s found each time

The second t y p e of i n t e r f e r e n c e is found when

Ana lys i s

The fo l lowing paper proposes t o l i s t t h e d i f f i c u l t i e s s p e c i f i c t o

a n a l y s i s of a r i d soils.

a r e shown on two s o i l samples.

General problems a r e reviewed and p r a c t i c a l examples ’

Only b a s i c d e t e r m i n a t i o n s a r e considered.

MATERIALS

Two c h a r a c t e r i s t i c a r i d s o i l s have been ana lyzed i n a c o l l a b o r a t i v e

s tudy by e i g h t e e n l a b o r a t o r i e s . S i x t e e n of them are c e n t r a l l a b o r a t o r i e s

of s o i l survey i n t h e i r own country.

a r e members of ACSAD, and they perform r o u t i n e work r a t h e r than r e s e a r c h .

All b u t two belong t o s t a t e s which

The f i r s t s o i l sample ( S o i l A) has been sampled i n t h e s u r f a c e horizon ,

-’ . 8 ,

/ . ”

<-- _ . . - . . . ._ . . . . .̂ . I !

. . - , I . ... - . __ . , -

I - 2 2 0 -

descr ibed a s Camborthic X e r a t h i c Holomorphic (ILAIWI). S o i l B be longs

t o a s i m i l a r p r o f i l e where gypsum accumulat ion is v i s i b l e as smal l c r y s t a l s .

Sampling has been done i n t h e gypsum accumulat ion zone between 40 and 65 cm.

Main c h a r a c t j e r í s t i c s a r e g i v e n i n T a b l e 1. The c o l l a b o r a t i v e s tudy inc luded

water and p l a n t a n a l y s i s n o t mentioned i n t h i s paper.

Table 1. Data on s o i l samples A and B.

F r a c t i o n (%I Clay S i l t : Sand Caco3 SP pH OM% NX ~~

Soil A 37 21 34 21.3 64 8.0 0.8 0.13 ' S o i l B 43 30 24 18.4 7a 8.1 0.1 0.04

m e q / 1-l Ca Mg K Na C 1 SO4 HC03 pH EC

S a t . e x t . A a 5.3 0.4 4.8 5.8 a 5.3 7.8 i..i

Sat:. e x t . B 31 26 0.8 130 96 92 1.6 7.6 16.6

Analys is C.E.C. Exch. N a P(0 lsen) Gyps. H O H O X meq/100 g meq/100 g ppm % 15'bar 1/3 bar

3.7 10 O 16.8 24.1 S o i l A 1 9 S o i l B 20 6.5 - 10.8 - -

METHODS

No s p e c i f i c recommendations have been made f o r us ing s p e c i a l methods;

soil samples A and B have been analyzed by methods used f o r r o u t i n e d e t e r -

mina t ions of s o i l parameters i n each l a b o r a t o r y .

a r e expressed 105'12 d r y b a s i s and s o i l B on a i r - d r y b a s i s . A l l t h e resul ts of s o i l A

1. Most l a b o r a t o r i e s used the s a m e type of method f o r a l m o s t a l l

t h e de te rmina t ions .

Procedures a r e d i f f e r e n t :

c a p a c i t y done by each l a b o r a t o r y us ing ammonium a c e t a t e a s a

s a t u r a t i n g s o l u t i o n and sodium a c e t a t e as a d i s p l a c i n g s o l u t i o n .

Procedures may i n c l u d e washing s t e p , c o r r e c t i o n f o r s o l u b l e salts ,

e tc . . . .

may be cons idered t o u s e a d i f f e r e n t method.

2. a t y p i c a l example is t h e c a t i o n exchange

The o v e r a l l p r o c e s s is so d i f f e r e n t t h a t each l a b o r a t o r y

It was not p o s s i b l e

i

I 1

1 1 i I

I I l i i

i 1 i

I ’ i

i ! i

1 !

i

!

f i i

i

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3

I

- 221 - t o i n v e s t i g a t e i n d e t a i l t h e mod i f i ca t ions adopted by each one.

Never the less , t h e need t o modify a proposed procedure is a c l e a r

s i g n of t h e p o t e n t i a l d i f f i c u l t i e s of a n a l y s i s of a r i d s o i l s .

The re i s no wonder, then , t o f i n d a wide v a r i a t i o n of r e s u l t s

around t h e mean v a l u e .

twofold:

f i l e s without a p r a c t i c a l p o s s i b i l i t y ‘of redoing them (survey a t

r e g i o n a l l e v e l fo’r i n s t a n c e ) , (2) P r e c i s i o n of r e s u l t must comply

w i t h a n a l y t i c a l requirements of c l a s s i f i c a t i o n systems.

consequences are p a r t i c u l a r l y important In U.S. S o i l Taxonomy

which r e l i e s on ve ry p r e c i s e a n a l y t i c a l l i m i t s .

Impl ica t ions of t h e s e v a r i a t i o n s are

(1) They must be considered when compiling r e s u l t s from

The

CRITERIA FOR EVALUATION OF RESULTS - - -

Twenty sub-samples d r a m from A and B r e s p e c t i v e l y have been t e s t e d

for homogeneity by analyzing t h e i r t o t a l sand conten t .

d e v i a t i o n f o r n=20 is 5% for A and 6% f o r B. a r e p r o p o r t i o n a l t o p a r t i c l e s i z e w e may consider 5% as t h e lowest p o s s i b l e

r e l a t i v e s tandard d e v i a t i o n (for n-20) r e p r e s e n t i n g t h e utmost compat ible

wi th sample homogeneity.

R e l a t i v e s tandard

I f w e assume t h a t many p r o p e r t i e s

On t h i s basis w e cons ider i n . t h i s paper ( for n=10):

- a n r.s.d. less than 10% as e x c e l l e n t .

- an r .s .d . l y i n g between IO-20% as good.

- a n r.s.d. between 20 and 30 a s poor.

- a n r . s . d . exceeding 30% a s ques t ionable .

O f course t h e s e v a l u e s are q u i t e a r b i t r a r y , but r .s .d . v a l u e is a oonvenient

c r i t e r i o n f o r eva lua t ing p r e c i s i o n .

a d i f f e r e n t approach not cons ide red i n t h i s paper.

Evaluat ion of accuracy is es t imated by

For each de termina t ion , t h e p o t e n t i a l sources of e r r o r s a r e l i s t e d

and practical examples a r e d i s c u s s e d on r e s u l t s of A and B samples.

, ” _.^. ”.- .. . __

!

! '

! i

i l i I .

- 222 -

PARTICLE SIZE DISTRIBUTION, SOURCES OF ERRORS

Procedures i n c l u d e p i p e t t e ( P i p e r , 19421, hydrometer (Day, 1950) ,

plumet (Marschal l , 1956).

and i n a l l c a s e s t h e f i r s t s o u r c e of misunderstanding l ies i n t h e d e f i n i t i o n

of p a r t i c l e .

gypsum are comprehended a s p a r t i c l e s or not :

calcium s u l f a t e d i h y d r a t e a p p e a r s i n t h e words c lay , s i l t and sand;

t h e o t h e r , i n s o l u t i o n a c r y s t a l of gypsum is l i k e l y t o f o l l o w S t o k e s '

l a w more c l o s e l y t h a n a n a g g r e g a t e of r e a l c l a y which may b e counted a s

f i n e sand.

f o r s o i l A.

All methods a r e based on S t o k e s ' l aw a p p l i c a t i o n

I n a r i d s o i l s w e m u s t a g r e e on whether ca lc ium c a r b o n a t e and

On t h e o n e hand n o t i n g such

On

S i g n i f i c a n t d i f f e r e n c e s a t t h i s l e v e l are d i s p l a y e d 'in T a b l e 2

Table 2. E f f e c t o f calcium c a r b o n a t e removal on e x p r e s s i o n of r e s u l t s :

S o i l A.

Caco3 Z Clay S i l t Sand (1) + (2) + (3) Caco3 s t a t u s '( 1 ) ( 2) (3)

37 27 34 98 a. Not removed 21.3

7 5 b. Removed 21.3 32 15 28

. 1 :

I n l i n e 5 calcium c a r b o n a t e is accounted f o r i n c l a y , s i l t and sand

f r a c t i o n and a g a i n a s ca lc ium carbonate .

though being t h e most common one, i s mis leading .

n o t g i v e any informat ion on t h e s i z e of Caco important a s p e c t . L i n e 1 p r e s e n t s d a t a n e a r e r t o r e a l i t y . Comparison

T h i s way of express ing r e s u l t s ,

Furthermore i t d o e s

p a r t i c l e s though it is a n 3

of l i n e s D and b shows t h a t calciunl c a r b o n a t e is mainly s i l t - s i z e d i n o u r

sample.

T h i s p re l iminary s o u r c e of i m p r e c i s i o n being c l e a r e d up, t h e main

s o u r c e s of problems a r e due t o :

a .

b.

I n a p p r o p r i a t e d i s p e r s i o n of t h e sample;

Devia t ions from t h e o r e t i c a l c o n d i t i o n s i n which S t o k e s ' l a w a p p l i e s ;

and -. c . Imperfect sampling of suspens ion o r e r r o r s d u r i n g weight ing

( p i p e t t e method).

Accumulation of t h e s e e r r o r s l e a d s t o a w i d e r v d r i a t i o n between labora-

. . I < - - i I ~ s ", I: , . . . ,

J- d- - 223 -

t o r i e s t h a n between methods, a r e l a t i v e s t a n d a r d v a r i a t i o n of 6% f o r

p i p e t t e and f o r plumet method as w e l l , being claimed f o r a sample having

284 c l a y (Bannis te r et al . , 1 9 7 3 ) .

i t s e l f as shown i n Table 3 .

Dispers ion method has no e f f e c t in

Table 3 . Dispers ion method and p r e c i s i o n of f l a y percentage (from

Edwards and Brenner, 1967; s o i l d a t a :

26.5%; Org. carb . 0 . 3 5 % ) .

pH 7 . 8 ; Caco., . .

,.

Dispers ion method Ul t ra -sonic Na-Polyphos. Res in Na-Hyeo- bromit e

Clay 4 2 s 24 .3 26.4 22.7

I n conclus ion , f o r s o i l s c o n t a i n i n g s o l u b l e salts, i f w e assume a

proper d i s p e r s i o n , r e g a r d l e s s t h e method used, t h e most i n s i d i o u s errors

a r e due to d e v i a t i o n s from Stokes ' law caused by:

a . Change i n v i s c o s i t y of s o l u t i o n due t o s a l t d i s s o l u t i o n o r temper-

a t u r e v a r i a t i o n dur ing t h e experiment;

P a r t i a l f l o c c u l a t i o n due t o excess of d o u b l e charged p a r t i c l e s ; b. C. Change i n time of sedimenting and/or d e p t h of sampling because of

d i f f e r e n t d e n s i t y of p a r t i c l e s (Table 4 ) ; n o t t o ment ion s u r f a c e

t e n s i o n e f f e c t , and n o n s p h e r i c i t y of p a r t i c l e s g r e a t e r than 5

m i c r o s i n d iameter , a l l f e a t u r e s i n h e r e n t t o t h e complex n a t u r e of

soils and n o t d i s t i n c t i v e of a r i d ones.

These e f f e c t s a r e f e l t d i f f e r e n t l y by each method. V a r i a t i o n of d e n s i t y

a f f e c t s t h e hydrometer method more t h a n t h e p i p e t t e method and t h e c l a y

f r a c t i o n (and s i l t ) more t h a n sand.

Table 4 . Devia t ion from Stokes ' l a w : e f f e c t of d e n s i t y .

E f f e c t on Real d e n s i t y E r r o r Mater i a 1 c l a y f r a c t i o n

2 . 2 +33.3 Montmor i l lon i te (dP2.3) Vermicul i te ( d 1 2 . 3 2 )

2.4 +14.1 Gypsum ( d 1 2 . 3 2 ) 2 .65 O NO 2 . 8 0 -1 1 . 1 C a l c i t e , do lomi te 2.90 -15.8 Underest imat ion

Over est h a t i o n

- 224 -

COMMENTS ON RESULTS ON SOILS A AND B

R e s u l t s are shown i n Tables 5 and 6. As expected, p r e c i s i o n is

poorer f o r c l a y and s i l t f r a c t i o n s t h a n f o r sand, t h e v a r i a t i o n being more

important f o r sample B (14% gypsum) t h a n f o r A (gypsum-free).

is determined b e t t e r than s u b f r a c t i o n s which a re a f f e c t e d by s i e v i n g e r r o r s .

S i n c e repea ted washing is necessary t o s e p a r a t e t o t a l sand, most e r r o r s

may b e avoided by choosing a sedimenta t ion t i m e s l i g h t l y l o n g e r than re-

quested. The 30% r . s .d . f o r sample B sand is probably d u e t o l o s s of

c r y s t a l l iz . i t lon wiitcr dur ing d r y h g and par tLa1 d l s s o l u t i o n of gypsum

s o l u t i o n .

T o t a l sand

Table 5. Sample A: R e s u l t s of p a r t i c l e s i z e d i s t r i b u t i o n (n = 12) .

. ( R e s u l t s expressed on 105°C b a s i s ; 80% of t h e r e s u l t s are

obta ined by p i p e t t e method,)

F r a c t i o n Clay S i l t T o t a l sand * F i n e sand Medium

Rang e 14-46 42-19 44-27 8-22 28-13 M&XATd 35 28 3 5 14 21 Stand. dev.(S) 9.5 8 - 2 4 -8 5.1 6.2

( s / m ) x l O O 27 29 14 36 30

'l'able 6. Sample B: Resulits of p a r t i c l e s i z e d i s t r i b u t i o n (n=lO). '. ( R e s u l t s g i v e n o n a i r d r y bas is . )

F r a c t i o n Clay S i l k T o t a l .sand F i n e sand Med. + Coarse . A

Range 12-70 48-11 38-1 9 7-1 3 4-14 Mean (m) 43 3 0 24 1 0 11 Stand.dev. (S) 1 6 12 7 2.3 4

(s /m)xloo 37 4 0 3 0 23 38

Few p r e c a u t i o n s have t o be taken i n c o n v e n t i o n a l procedures w i t h

g y p s i f e r o u s samples, e s p e c i a l l y when gypsum exceeds , 5 % :

- C o r r e c t i o n of S tokes ' c o n s t a n t a c c o r d i n g t o gypsum c o n c e n t r a t i o n

( V i e i l l e f o n , 1976) ( c f . Table 7 ) .

- P a r t i a l i n s o l u b i l i z a t i o n of gypsum by barium c h l o r i d e (Bascomb,

Hesse, V i e i l l e f o n , Matar, Douleimi.. . .) . - I n v e r s i o n of sampling o r d e r t o d iminish gypsum d i l u t i o n e f f e c t .

- Drying p r e c a u t i o n (Viei lBefon) .

1.

i

- 225 -

Table 7 . Sedimentat ion t ime f o r c l a y and si l t sampling a t 10 cm

d e p t h accord ing to gypsum content ( i n minutes and seconds) .

Gypsum percentage O 10 20 30 40 50

Temperature 20" 4 ' 4 0 4 ' 4 6 4 ' 5 2 4 ' 5 0 5 '04 5'11 Temperature 25" 4 ' 0 8 4 ' 1 3 4 ' 1 9 4 '24 4 ' 3 0 4 ' 3 6

CALCIUM CARBONATE

A l 1 methods are based on t h e r e a c t i o n of calcium c a r b o n a t e w i t h d i l u t e d ' hydrochlor ic a c i d and t h e subsequent measure o f :

a. Volume of carbon d i o x i d e evolved (volumetr ic c a l c i m e t e r ) ;

b. Back t i t r a t i o n of a c i d ( ac id t i t r a b l e b a s i l e t y ) ;

C. LOSS of weight t o d i s p a r i t i o n of CaCO ( g r a v i m e t r i c ) .

E r r o r s may a r i s e from a n improper c a l i b r a t i o n of calcimeter, o r s o l u b i l i - 3

z a t i o n of Co2 i n a c i d and manometric l i q u i d . In a d d i t i o n a l l methods have i n ccm"mn a l o w r e a c t i o n r a t e when dolomi te is p r e s e n t ( t h e la t ter being counted

f o r a s pure calcium carbonate) and i n t e r f e r e n c e of MnO composi t ion by a c i d ( t h e l a s t p o s s i b i l i t y being unusual i n a r i d zones) .

A l k a l i n e e a r t h c a r b o n a t e measured from a c i d n e u t r a l i z a ì i o n is more

and o r g a n i c m a t t e r de- 2

dependent on s o i l / a c i d ratio and a c i d c o n c e n t r a t i o n .

p r a c t i c a l a p p l i c a t i o n fo r a c c u r a t e de te rmina t ion of low ca lc ium c a r b o n a t e

percentage (Miyamoto e t a l . , 1 9 7 3 ) .

T h i s method f i n d s i ts

1

Rapid g r a v i m e t r i c method i s claimed t o y i e l d r e s u l t s n o t vary ing more

t h a n 1% on t r i p l i c a t e a n a l y s i s i n t h e o v e r a l l range of O t o 70% ca lc ium

c a r b o n a t e (Bauer e t a l . , 1 9 7 2 ) .

T a b l e 8. Calcium c a r b o n a t e r e s u l t s .

Sample A B ~ _ ~ _

n 9 7

Mean m 21 18 Range 18.5 t o 24.4 14 .8 to 2 1 . 4

Stand. dev. 1.7 2 . 2 (s/m)x 100 a 12

..'._.. , ,'.. .

. _ _ . I . , " .. . - ~ _ . _ .. -. . %..=:y.- . .. , ... . . - .

. 3 .e. , .

. .

- 226 -

DETERMINATION OP OKCANIC MATTER AND NITROGEN

I l ’ ~ O ~ J ~ & ! i l i : i I I I wi ; : t i i lc i i u t ter ciad n l t r o y c n dctcriiiLiiiiL1uii iirc jir!ii&!rully

found i n h i g h l y o r g a n i c s o i l s n o t common i n a r i d zones.

I n t e r f e r e n c e of c h l o r i d e , which is oxid ized by potassium dichromate

(methods Walkley & Black and method Anne) is n o t e f f e c t i v e f o r c o n c e n t r a t i o n

up t o 2400 ppm c h l o r i d e (Viswanatman) and may be c o r r e c t e d by s i l v e r s u l f a t e

i f necessary . Standard d e v i a t i o n a t 0.72% O.C. l e v e l and n=9 i s 13’:

o n l y f o r sample A.

. , DETERMINATION OF SOLUBLE SALTS

E v a l u a t i o n of s o l u b l e salts is commonly done by

by s y s t e m a t i c 1/1 o r 1/2.5 e x t r a c t s .

s a t u r a t i n g extract o r

- Making a s a t u r a t i o n p a s t e is a s i m p l e procedure: e r r o r s a r e ex-

pec ted w i t h f i n e - t e x t u r e d soils, e s p e c i a l l y w i t h s w e l l i n g c l a y s and

h igh scldium content . Though e s t i m a t i o n of s a t u r a t i o n p o i n t is l e f t

t o personnel a p p r e c i a t i o n , t h e s a t u r a t i o n percentage c o u n t s among

t h e most r e p r o d u c i b l e procedure (r .s .d . 6%).

T a b l e 9. Determinat ion of s a t u r a t i o n e x t r a c t sample A (n=12). ~

Mean Rang e V a r i a b i l i t y (as r.s.d.L)

S a t u r a t i o n (%) 64 57-70 6 Elec . Conduc. (mS cm-l) 1.7 1.3-2.3 1 6 Sum of i o n s (meq/e) (Cat ions i l \nions) /2 19.6 17-27 13

J .

The s a m e cannot be s a i d of t h e subsequent e x t r a c t i o n of a s o i l

s o l u t i o n .

range of e l e c t r i c c o n d u c t i v i t y is g r e a t e r t h a n t h e s a t u r a t i o n percentage .

Suggested r e a s o n s could be:

The r e l a t i v e s tandard d e v i a t i o n as w e l l , a s t h e v a r i a b i l i t y

- Though adding t h e same q u a n t i t y o f water t o r e a c h s a t u r a t i o n ,

s t i r r i n g may b r i n g i n t o s o l u t i o n d i f f e r e n t q u a n t i t i e s of s a l t s

( r .s .d . 162).

i

I i

1

i f ?

I l i ,

- 227 -

- T h e of c o n t a c t between s o i l and s o l u t i o n , and tempera ture a s w e l l ,

d i f f e r s from one o p e r a t o r t o a n o t h e r , ' a n d from p l a c e t o p l a c e .

- E x t r a c t i o n by s u c t i o n on c e n t r i f u g a t i o n may e x t r a c t more o r less

of t h e s a t u r a t i n g water (cf. T a b l e '9).

I O N I C COMPOSITION OF SATURATION EXTRACT

Table 10. V a r i a t i o n s i n i o n d e t e r m i n a t i o n o f A, B and water sample.

Ca Mg K Na C 1 SO4 Notes Sample

A 14 13 1 0 14 12 1 0 V a r i a t i o n

(s/m)x100 26 38 23 51 35 32 e x t r a c t i o n

( s / m ) x l O O 25 31 63 22 25 . 38 V a r i a t i o n B mean 29 26.5 1 . 0 134 96 70 due t o

A Mean 8.3 5.2 0.4 5 .5 5.5 7 .9 d u e t o

B - - - e x t r a c t i o n

Water r . s . d . 24 27 25 14 1 9 38 V a r i a t i o n Average con- due t o Water

c e n t r a t i o n 14 16 0.7 41 56 11 method

A comple te review of a n a l y t i c a l problems f o r each element is o u t of

o u r scope, b u t a few remarlcs a r e worthwhile t o mention:

- a. A n a l y s i s of s tandard water sample was done t o g e t h e r w i t h s o i l s A

and B.

averaging r e s u l t s of A and B (see Table 10) .

r e s u l t s depends on methods used t o de te rmine each element.

V a r i a t i o n i n a n a l y s i s of samples A and B is g r e a t e r because of t h e

e x t r a c t i o n process .

The h igh v a l u e found f o r potassium is probably due t o sodium

i n t e r f e r e n c e .

As f a r a s . c l a s s i f i c a t i o n o f s o i l s is concerned, t h e most apparent

d i f f i c u l t y l ies i n t h e a p p l i c a t i o n of t h e sodium a b s o r p t i o n r a t i o

concept .

s a t u r a t i o n e x t r a c t is 2.4 meq/l- ' .

(Richard ' s Formula) exchangeable sodium v a l u e 2.2 meq/100 g ,

whi le t h e average found by a n a l y s i s i s 3.7.meq/100 g.

_ .

Water sample r e f e r r e d t o has a c o n c e n t r a t i o n i n a l l e lements

V a r i a t i o n of water

b.

c.

d.

SAR c a l c u l a t e d from mean v a l u e s of Ca, Mg and Na i n

T h i s v a l u e g i v e s a c a l c u l a t e d

. . . , , . . . . - _ _ *--- - ._ y.- - - . - .. .- _- .--- -- .: i- . - - ~ .. ' .

, :'

!.

- 220 -

I

EXCHANGEABLE CATION AND CATION EXCHANGE CAPACITY

Determinat ion of c h a r a c t e r i s t i c s of t h e exchangeable complex of a r i d

s o i l s i s a c t u a l l y t h e most t roublesome a n a l y t i c a l p rocess .

worse, no r e f e r e n c e procedure e x i s t s and p o s s i b l e c o r r e l a t i o n of c a t i o n

exchange c a p a c i t y e i t h e r w i t h c l a y and o r g a n i c m a t t e r p e r c e n t a g e o r air- d r y water c o n t e n t , o r s a t u r a t i o n percentage , a r e not a p p l i c a b l e i n s o i l s

c o n t a i n i n g a p p r e c i a b l e amounts of calcium c a r b o n a t e and gypsum.

To malce t h i n g s

Because exchangeable c a t i o n s a r e t o be removed from soil exchange

sites p r i o r to d e t e r m i n a t i o n exchange capac i ty . t h e prohlems f o r de te rmining

CEC and exchangeable c a t i o n s , sodium f o r i n s t a n c e , a r e s i m i l a r and c l o s e l y

l i n k e d .

w i l l be a f f e c t e d by a double source of imprec is ion:

t h e second due t o exchangeable Na e v a l u a t i o n .

E r r o r s a r e l i k e l y t o accumulate and exchangeable sodium p e r c e n t a g e

t h e f i r s t d u e t o CEC,

Cat ion exchange i n c l u d e s a b s o r p t i o n on exchange s i t e s of c l a y , chemical.

r e a c t i o n o n f u n c t i o n a l groups o f o r g a n i c m a t t e r , and a d s o r p t i o n on t h e s u r f a c e

of c r y s t a l l i n e p a r t i c l e s .

physico-chemical p o i n t o f view.

Each of t h e s e phenomena is d i f f e r e n t from a

Genera l ly , t h r e e t y p e s o f problems e x i s t :

a .

b.

C.

Ca t ion exchange is d e f i n e d r e l a t i v e t o a g i v e n exchanger and n o t

Those i n h e r e n t t o t h e d e f i n i t i o n of c a t i o n excbange c a p a c i t y ;

Those i n h e r e n t t o t h e method used; and

Those i n h e r e n t t o t h e technique.

a b s o l u t e l y .

exchangeable sodium e x t r a c t e d accord ing t o t h e e x t r a c t i n g s o l u t i o n .

(Ammonium a c e t a t e e x t r a c t i n g less sodium than barium c h l o r i d e f o r i n s t a n c e . )

The r e l a t e d e r r o r is a d i f f e r e n t CEC o r a d i f f e r e n t amount of

Dcf i n i t ion

Cat ion exchange is not a n independent process . Permanent e q u i l i b r i u m

between s o l u b l e and exchangeable forms of c a t i o n s t a k e s p l a c e , making it

d i f f i c u l t t o s e p a r a t e a s p e c i f i c form. This r e l a t i o n s h i p has been demon-

s t r a t e d by Richard e t a l . y = 0.0145; x = 0.0126;

and r = 0.923 where y s t a n d s f o r exchangeable sodium r a t i o (exchangeable

complex); x f o r sodium a d s o r p t i o n r a t i o ( s o i l s o l u t i o n , i .e. s a t u r a t i o n

e x t r a c t ) .

c a t i o n s l e a d s t o t h r e e t y p e s of e r r o r s :

(1954) w i t h equat ion:

The i n t e r r e l a t i o n s h i p between s o l u b l e and exchangeable forms of.

f i i 1 1

1

i i t i

a T

T

E

3

i i i i 1

i r i

i 1

- i t t f

j

1 f

i , , i

t i 3

- 229 -

a . An o v e r e s t i m a t i o n of exchangeable c a t i o n s , by d i s s o l u t i o n of

calcium carbonate , calcium s u l f a t e and/or sodium c h l o r i d e .

The e x t r a c t i n g s o l u t i o n d i s s o l v e s d i v a l e n t c a t i o n s , and i n t r o -

duces i n t h e s o i l sample an a r t i f i c i a l compet i t ion between calcium

and sodium, f o r i n s t a n c e .

where sodium is r e p l a c i n g c a t i o n .

S u b s t r a c t i o n of s o l u b l e s a l t s as found i n s a t u r a t i o n e x t r a c t s ,

a p a r t from sodium h y d r o l y s i s , a d d s a n o t h e r s o u r c e of e r r o r :

t h e v a r i a t i o n of sodium found i n s a t u r a t i o n e x t r a c t ( c f . T a b l e 9).

b.

-_ An incomplete replacement is found

c .

Method used

The second group of e r r o r s a r i s e s from t h e method used. Though many

have been proposed, t h e o r i g i n a l ammonium a c e t a t e method (Bower,1952) i s

, o f t e n r e f e r r e d to as "convent ional" o r "s tandard." The shortcomings of

:. t h i s method have been o u t l i n e d (Okazaki et a l . , 1964; Polemio and Rhoades,

. 1977). a . In t h e s a t u r a t i o n s t e p (by sodium a c e t a t e pH: 8 . 2 ) :

- Colciuur d i s s o l v e d from carbonate .ond gypsum competes w i t h sodium

f o r occupying exchange sites and t h e s i t u a t i o n is incomplete .

b. In t h e washing s t e p (by a minimum q u a n t i t y of .? thanol) .

- Ion compet i t ion and sodium h y d r o l y s i s o c c u r s i f washing is

top e n e r p i c .

- Incomplete removal of s a t u r a t i n g sodium s o l u t i o n h e l d by c a p i l -

l a r i t y o c c u r s i f washing i s t o o mild. I

- c. In t h e replacement s t e p (normal and n e u t r a l amonium a c e t a t e s o l u t i o n ) .

- Incomplete replacement of N a by ammonium occurs .

Technique used

The t h i r d group of d i f f i c u l t i e s , and n o t t h e l e a s t , comes from t h e

technique used. Two systems a r e p o s s i b l e : l e a c h i n g (column), o r mixing--

c e n t r i f u g i n g (ba tch) . Sources of e r r o r s a r e : : - Improper s o i l s o l u t i o n c o n t a c t because of channel ing (column). . ' - Loss o f excb-angeable m a t e r i a l by over -d ispers ion d u r i n g washing

s t e p ( i f c e n t r i f u g a t i o n speed is t o o slow) o r d i f f i c u l t y i n r e -

d i s p e r s i n g s o i l ( i f speed i s t o o high) ( b a t c h method).

b '

i

/ j

I 1 i 1

i i

I.

- 2 3 0 - Table 11. CFC. R e s u l t s of A and B samples.

n S Mean r . s .d . range

il 9 . 19 , 4.6 24 11 to 26

B 8 21 10.8 51 10 t o 35

i

EXCHANGFABLE CATIONS

Exchangeable c a t i o n s may b e determined i n t h e f i r s t s t e p of CEC i n

a n excess of s a t u r a t i n g s o l u t i o n , provid ing t h e s a t u r a t i n g c a t i o n does n o t

p r e e x i s t a s a n exchangeable one.

T h e o r e t i c a l l y , s a t u r a t i n g c a t i o n s , a l s o c a l l e d index c a t i o n s , should

l ime il g r m t c r :iTTinity Tor cxclinngc s i t e s than Ca, My, Na, K. Thr s a t u r -

., . . - ~

a t i n g s o l u t i o n should n o t , i n a d d i t i o n , d i s s o l v e s o l u b l e s a l t s .

t h e s e c o n d i t i o n s i t should have a very low d i e l e c t r i c c o n s t a n t , i n o r d e r t o . l e a v e sodium salt undisso lved .

t i o n , t h e index c a t i p n being f i x e d by a c o v a l e n t bond.

a very weak a f f i n i t y f o r exchange sites.

To f u l f i l l

I n o t h e r words it should be a n o r g a n i c so lu-

It has consequent ly

- S e v e r a l approaches were made by w i n g mixtures of organic and in-

o r g a n i c s o l e a i o n s ( a l c o h o l i c ammonium chloride--Tucker, 1971;

t r i e t h a n o l a m i n e s o l u t i o n s such a s barium chloride--Mehlich, 1948;

Bascomb, 19631, b u t none i s f r e e from e r r o r s .

l

The f i r s t method d o e s n o t e l i m i n a t e t h e need f o r washing 'out s o l u b l e

sodium s a l t s (by g l y c o l , e t h a n o l t o avoid h y d r o l y s i s of Na).

second one i s a f f e c t e d by a secondary r e a c t i o n of barium w i t h gypsum

The

and calcium carbonate . . -" We may expect a very low l e v e l o f p r e c i s i o n and accuracy f o r CEC and - - I

j . exchangeable c a t i o n s , Ithe e f f e c t of e f r o r s being g r e a t e r f o r Ca and Na,

lower f o r K and Mg.

a b l e sodium.

We have d i s c u s s e d above t h e low p r e c i s i o n of exchange-

Complete r e s u l t s a r e shown i n Tables 12 and 13.

i . .-I.

t. l

- 231 -

Table 19. Sample A: .exchangeable sodium.

No. EN^ (meq/100 g)" ESPb E x t r a c t i o n C o r r e c t i o n

A 23 o. 5 4 NH40Ac

A 9 1.08 5 NH40Ac Minus Na i n s a t .

A 31 1.5 8 NH,OAc .ext. A 10

A 3 1.5

3.1

L)

8 n.i.a Corr. f o r sol.

27 NH , OAc 4

6.37 34 n . i . a

11.7 44 NH4 OAc

A 21

A 13

- mean (n=7)

s tand . d e v i a t i o n

3.7 18.4

Does not follow Gauss c u r v e

aENa = is g i v e n a s meq/100 g 105°C d r y s o i l

z bESP = (ENa x 100)/C.E.C.

Table 13. Sample B: Exchangeable sodium.

Sample No. ENaa ESP E x t r a c t i o n C a l c u l a t i o n s

B 20 3 .9 21 n. i . a . n . i .a .

NI1 OAcpH 7.0 n. i .a . 4 B 30 4.0 22

B 3 4.2 12 NH40AcpH 7.0 Minus N a i n s a t . e x t .

Minus Na i n s a t . e x t .

B 22 8.3 79 NH'OAc Minus N a i n s a t .

5 . 9 37 n . i . a . n.1.a. - B 11

. B 8 7.0 63 NH40Ac

e x t . B 12 12.2 4 2 NH40Ac n.i.a.

- - mean v a l u e (n=7) 6.5 39

D i s t r i b u t i o n d o e s n o t follow a Gauss curve

'ENa = is g i v e n as meq/100 g 105°C d r y s o i l .

b . n . 1 . a . = no i n d i c a t i o n s a v a i l a b l e

..

.

- 337 - PHOSPHORUS

The measure of l i a b i l i t y of s o i l phosphorus is a f f e c t e d l i k e t h e

measurement of CEC, by a l a c k of a b s o l u t e d e f i n i t i o n .

by t h e q u a n t i t y e x t r a c t e d by a g i v e n method when i t c o r r e l a t e s w i t h p l a n t

absorp t ion i n f i e l d or pot experiment.

e x i s t s a t p l a n t l e v e l and a t t h e e x t r a c t i n g s o l u t i o n l e v e l .

L i a b i l i t y i s assessed

Imprec is ion i n t h e d e f i n i t i o n

Inaccuracy i s due t o i n c o r r e c t c h o i c e of method w i t h regard t o s o i l

p r o p e r t i e s . Imprec is ion i s due t o nonrespec t of s tandard c o n d i t i o n s w h a t -

ever method is used, f o r t h e same e x t r a c t i n g s o l u t i o n , ( i . e . same r e a g e n t ,

same concent ra t ion , same pH. ..l. among o t h e r s :

D e v i a t i o n s from s t a n d a r d procedure may be,

- S o i l s t o r a g e time: a v a i l a b l e phosphate may i n c r e a s e by 14% a c c o r d i n g

t o Nowosielski ( c i t e d by Bat ten , 1978).

- Time and speed of shaking s o i l w i t h reagent .

- Time and mode of c o n t a c t between s o i l and s o l u t i o n , s o i l s o l u t i o n

r a t i o . These remarks apply f o r each r e a g e n t .

To mention o n l y t h e methods u s i n g chemical r e a g e n t s i n c a l c a r e o u s s o i l s :

- NaHC03 pH 8.5 (Olsen) . - NaOH N/10 (Saunder) .

- NaHC03 NH4 pB 8.5 (Olsen-Dabin) . C o l l a b o r a t i v e s tudy on s o i l A gave a v e r y c l o s e agreemenc on liable

phosphorus (NaHC03 pH.8.5 f o r 4 d e t . on ly) w h i l e t h e r e s u l t s of t o t a l

s ~ . ) * phosphorus show a g r e a t e r d i s p e r s i o n .

Table 14. “Avai lable“ and t o t a l phosphorus, sample A . . -

Sample A l A4 A l 3 A14 - n mean

Avai l . P (ppm) 11 9 1 0 1 0 - 4 10 Sample A3 A5 A ? A10 A23 - - T o t a l (ppm) 1600 2190 1830 3240 8901 5 1950 Y

Method HCL04 HCL04 HN03 - 4N03 - - ‘

(Note t h a t the high v a l u e s of t o t a l phosphorus a re probably d u c t o

f e r t i l i z e r s ) . 1.

I ,

1

i.

- 2 3 3 - ' DETERMINATION OF GYPSUM

Table 15. Sample B: gypsum percentage.a

No. Gyp=m (XI ' . Method

B 11 5.3 n.d.a.

B 19 9.8 n.d.a.

B 5. 10.75 D i f f e r e n c e i n Acetone preci$tates of 1/20 and s a t u r a t i o n extract

B 22 11.2 P r e c i p i t a t i o n by Acetone B 8 12.5 Dissolved i n b o i l i n g (N1i4)2 COj/

g rav imet ry BaC1, - ~

B 12 14.2 Acetone/Elec t r i c c o n d u c t i v i t y

B 13 . 14.5 n.d.a.

B 17 14.7 n.d.a.

11.6 mean v a l u e (n=10)

3.1 s t a n d a r d d e v i a t i o n

27 r e l a t i v e s tandard d e v i a t i o n %

aGypsum percentage is g i v e n on a i r - d r y weight b a s i s .

Comments

I f t h e r e s u l t s a r e lower t h a n t r u e v a l u e , it may come from a t o o

h i g h s o i l / s o l u t i o n r a t i o , complete d i s s o l u t i o n o f gypsum n o t being achieved

( c f . Table 16) .

Table 16. D i s s o l u t i o n of gypsum: s o i l s o l u t i o n r a t i o s i n water (based on 2.6 q/1 s o l u b i l i t y , from Hesse).

Gypsum content of s o i l s % 0-1 1-2 2-5 . 5-10 10-20

Soi l /Solu t i o n r a t i o 1/5 1 /10 1 /20 1 /50 1/100

Otherwise t h e r e i s no d i f f i c u l t y and p r a c t i c a l l y no p o t e n t i a l s o u r c e

of e r r o r i n gypsum determina t ion , though r e s u l t s cannot b e expressed on

oven-dry b a s i s .

- A r a p i d method, based on loss of weight by c o n t r o l l e d hea t ing and

r e h y d r a t i o n makes i t p o s s i b l e t o measure both gypsum c o n c e n t r a t i o n

(wi th a c e l a t i v e s tandard d e v i a t i o n of 5Z) and hvdroscouic water

. .

i.

- 2 3 4 -

c o n t e n t ( V i e i l l e f o n , 1978).

CONCLUSIONS

We can make two k inds of remarks t o conclude. The f i r s t is in tended

a n a l y s t s i n each l a b o r a t o r y must e v a l u a t e , t o t h e s t a f f of l a b o r a t o r i e s :

time t o t ime, and f o r each method: i t s p r e c i s i o n (measured as r e l a t i v e

s t a n d a r d d e v i a t i o n of t e n r e p l i c a t e s a t t h r e e d i f f e r e n t l e v e l s of t h e range

covered by t h e method) and i ts accuracy (es t imated as t h e d e v i a t i o n from

t h e " t r u e value" of a r e f e r e n c e sample) .

I n a d d i t i o n t o t h a t , h e must d e t e c t p a r a - a n a l y t i c a l s o u r c e s of e r r o r s ,

p a r t i c u l a r l y . important i n s o i l a n a l y s i s ( s o i l sample p r e p a r a t i o n , s t o r a g e

and subsampling).

a . By mul t ip le -cross checking of r e s u l t s u s i n g approximate r e l a t i o n -

s h i p s between s o i l p r o p e r t i e s , f o r i n s t a n c e :

- Clay %.versus water r e t e n t i o n , c l a y % + silt % + 0 . C X v e r s u s CEC.

- I o n i c ba lance i n s o l u t i o n s .

- I o n i c c o n c e n t r a t i o n and e l e c t r i c c o n d u c t i v i t y (a l lowance being CA NA made f o r - and/or S04/C1 r a t i o s ) .

- Carbonate b i c a r b o n a t e and pH.

- P r o p o r t i o n a l i t y between ESP and SAR. - R e s u l t s of a n a l y s i s should b e expressed w i t h a number of d i g i t s

i n accordance w i t h t h e v a r i a t i o n c o e f f i c i e n t of r e p l i c a t e

d e t e r m i n a t i o n on a r e f e r e n c e sample.

b. By u s i n g methods dim,inishing human c o e f f i c i e n t of e r r o r s ( f o r

i n s t a n c e water s a t u r a t i o n percentage by c a p i l l a r i t y procedure

(Longenecker) r a t h e r than hand mixing and s t i r r i n g ) .

By reducing t h e number of s t e p s of e a c h method t o a r e a s o n a b l e

minimum ( f o r i n s t a n c e two s t e p s CEC i n s t e a d o f t h r e e , e l i m i n a t i n g

e r r o r s due t o washing s t e p ) .

By l i m i t i n g r e s u l t s ob ta ined by d i f f e r e n c e .

c.

d .

The second kind of remark i s d i r e c t e d t o t h e p o t e n t i a l u s e r s of r e s u l t s .

A g r e a t d e a l of a n a l y s i s has been made through t h e world and i t s compi la t ion

might be u s e f u l a t a regiomal l e v e l f o r mapping, f o r e v a l u a t i n g s a l i n i t y

.. . .

' : . 2

SOIL,SCIENCE DIVISION

: -C$

ACSAD / SS / P17 / 1981

PROCEEDINGS TERNATIONAL ( .

I AMASCUS 1981

THE ARAB CENTER FOR THE<STUDIES OF ARID ZONES AND DRYLANDS ( ACSAD )

1